A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A powertrain of such an automatic transmission that includes the plurality of planetary gear sets changes rotating speed and torque received from a torque converter of the automatic transmission and accordingly changes and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a powertrain such as durability, efficiency in power transmission, and size depend a lot on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a powertrain are also under constant investigation.
A manual transmission that has too many speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically without needing manual operation.
In addition to various developments regarding four and five speed powertrains, powertrains of automatic transmissions realizing six forward speeds and one reverse speed have recently been introduced, examples of which are found in U.S. Pat. No. 6,071,208 that was issued on Jun. 6, 2000, and in U.S. Pat. No. 5,226,862 that was issued on Jul. 13, 1993.
The powertrain discussed in U.S. Pat. No. 6,071,208, for example, includes a double pinion planetary gear set and a pair of single pinion planetary gear sets. A first carrier is fixedly connected to an input shaft, and a second carrier always operates as an output element. The disclosed powertrain includes six friction elements of two clutches and four brakes. However, it is preferable to use fewer friction elements to enable six forward speeds and one reverse speed so that an automatic transmission can be more light and compact.
The powertrain disclosed in U.S. Pat. No. 6,071,208 operates first and fourth brakes at a first speed, a first clutch and the first brake at a second speed, a second clutch and the first brake at a third speed, the first and second clutches and at a fourth speed, the second clutch and the fourth brake at a fifth speed, and the second clutch and the third brake at a sixth speed, respectively. The second and fourth brakes and are operated at a reverse speed.
The design of this powertrain, however, leads to a number of disadvantages. For example, at the third forward speed, a first sun gear rotates at a speed of more than twice that of the rotation speed of the input shaft. In addition, the slip speed of the fourth brake which is not operated in the third speed, becomes as high as that of the first sun gear 18. The third forward speed is frequently engaged in the case that acceleration is needed, since a six-speed automatic transmission usually achieves the speed ratio of 1:1 at the fourth forward speed. Therefore, durability of an automatic transmission deteriorates if an element always rotates at a high speed in such a shift speed.
Also, slip speeds of friction elements are excessive for all speed ranges, which deteriorates durability of the automatic transmission and also causes excessive power loss. Therefore, the powertrain could be improved to have lesser slip speeds of friction elements for speeds D2–D6. In particular, the sum of slip speeds of friction elements becomes excessively large at the sixth forward speed D6, and therefore, the durability problem is at its maximum at the sixth forward speed.
In addition, when considering the number of planetary gear sets that take part in power transmission, at least two planetary gear sets take part in the power transmission for the fifth and sixth speeds, which deteriorates power efficiency. It is preferable that efficiency of power transmission is improved.
The six speed powertrain disclosed in U.S. Pat. No. 5,226,862 includes a double pinion planetary gear set and a pair of single pinion planetary gear sets. The input shaft is variably connected to second and third sun gears that are fixedly interconnected, a second carrier and a first sun gear, through first, second, and third clutches, respectively. A third carrier always operates as an output element, and a first carrier is fixed to the transmission housing 7. Also, first and second ring gears and the second carrier and a third ring gear and the second and the third sun gears are fixedly interconnected, respectively. The powertrain further includes a first brake that can stop rotation of the fixedly interconnected second carrier and third ring gear and a second brake that can stop the fixedly interconnected first and second ring gears.
This powertrain perates the first clutch and the first brake at a first speed, the first clutch and the second brake at a second speed, the first clutch and the third clutch at a third speed, the first and the second clutches at a fourth speed, the second and third clutches at a fifth speed, and the second clutch and the second brake at a sixth speed, respectively. The third clutch and the first brake are operated at a reverse speed. In particular, at the fourth speed, the first sun gear rotates at a speed of more than twice that of the rotation speed of the input shaft. The third forward speed is frequently engaged in the case that acceleration is needed, since a six-speed automatic transmission usually achieves the speed ratio of 1:1 at the fourth forward speed. Therefore, durability of an automatic transmission deteriorates if an element always rotates at a high speed in such a shift-speed.
Furthermore, at the fourth speed, the speed of a first planetary gear (i.e., pinion gear of the first planetary gear set) is almost 3.8 times that of the input shaft. This kind of high relative revolution speed may cause critical harm to durability of planetary gear sets. So, it is believed that this powertrain is not appropriate for use for automotive vehicles unless the durability is substantially supplemented.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.